This is not a function implemented in the current IDL math analysis libraries. I had hopes that I would find it in IDL's fairly new interface to the IMSL C libraries. I ***do*** find an IMSL_HYPERGEOCDF function, which evaluates the hypergeometric distribution function. IDL's IMSL_RANDOM can also use the hypergeometric function (using keyword /HYPERGEOMETRIC) in its random number generation. I suspect this is not what you are looking for, but these are in any case two options you can check out. James Jones